Problem #103: Binary Tree Zigzag Level Order Traversal

Intuition & Strategy Breakdown

Use BFS level order traversal. Maintain a direction boolean toggle to reverse level array order on alternating tree levels.

Python Solution

class Solution:
    def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root:
            return []
        
        res = []
        q = collections.deque([root])
        left_to_right = True
        
        while q:
            level_size = len(q)
            level = []
            for _ in range(level_size):
                node = q.popleft()
                level.append(node.val)
                if node.left: q.append(node.left)
                if node.right: q.append(node.right)
            
            if not left_to_right:
                level.reverse()
            
            res.append(level)
            left_to_right = not left_to_right
            
        return res

Java Solution

class Solution {
    public List> zigzagLevelOrder(TreeNode root) {
        if (root == null) return new ArrayList<>();
        
        List> res = new ArrayList<>();
        Queue q = new LinkedList<>();
        q.add(root);
        boolean leftToRight = true;
        
        while (!q.isEmpty()) {
            int size = q.size();
            List level = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = q.poll();
                level.add(node.val);
                if (node.left != null) q.add(node.left);
                if (node.right != null) q.add(node.right);
            }
            if (!leftToRight) Collections.reverse(level);
            res.add(level);
            leftToRight = !leftToRight;
        }
        return res;
    }
}